Three
astronauts on the International Space Station are set to return to Earth tonight
(Sept. 24) ? a full day late - but not because of problems with their Soyuz
landing capsule. It was a set of stuck hooks on the station that thwarted their
first homecoming attempt.

But how did
the unprecedented undocking glitch happen in the first place? Russian engineers
narrowed the list of electronic suspects to a faulty microswitch that may have
caused an open circuit in the docking port's wiring.

At the core
of the malfunction is a set of hooks and latches on the space station's Russian
Poisk docking port, which refused to release the astronauts' Soyuz capsule
last night.

The glitch
was fixed overnight and the astronauts will try to leave the space station
again tonight at 10:02 p.m. EDT (0200 GMT) and land
early Saturday on the steppes of Kazakhstan in Central Asia.

The
microswitch issue popped up early in Thursday night's undocking preparations
when astronauts had trouble confirming the hatch between the Soyuz
capsule and space station had a tight seal. After extended leak checks, however,
the astronauts pressed ahead with their undocking plans.

What
happened next was unexpected.

Russian
flight controllers were unable to command the hooks and latches on the Poisk
module to release the Soyuz spacecraft. The Soyuz has its own system of hooks
and latches as well, but the ones on the Poisk module must be released first. [Graphic ? Inside and Out: The
International Space Station]

"Basically,
two sets of hooks close around each other in a docking interface," NASA
spokesman Rob Navias told SPACE.com. "Last night, the microswitch
indicator was not providing any feedback that it was receiving electrical
continuity through the circuitry."

This was the
same component that had trouble confirming the seal between the Soyuz capsule
and Poisk module, Navias said. At the moment, engineers are unsure if the two
hiccups are related.

"It's
possible," Navias said. "It's not conclusive at this point but the
data certainly point to a similar issue that would lead back to that microswitch
indicator."

The glitch
caused hours of delays and, eventually, forced Mission Control and the
astronauts to scrap undocking and landing plans for a day. The Soyuz crew ?
NASA astronaut
Tracy Caldwell Dyson and Russian cosmonauts Alexander Skvortsov and Mikhail
Kornienko ? returned inside the space station, and 12 Russian recovery
helicopters awaiting their landing in Kazakhstan went back to their home
airfields.

Overnight,
the space station crew rigged up cables to fix the docking port to allow
tonight's Soyuz departure.

"Space
station crew members installed a series of jumpers, bypassing a failed
component that had prevented commands from being received by the Russian Poisk
module's docking mechanism," NASA officials said in a status update.
"Once the jumpers were in place, the Poisk module hooks and latches were
successfully opened."

The
hook-and-latch system on the Soyuz remains closed, securing the spacecraft to
the station, they added.

The space
station's Poisk docking port is on the top of the orbiting
laboratory's Russian segment. It was delivered in 2009 and entered use for
visiting Russian spaceships earlier this year.

This will be
the first undocking conducted at the Poisk module, so Russian engineers will be
closely analyzing the problem, Navias said. The Poisk module will also be the
same port that the next trio of space station crewmembers arrive at on Oct. 10.

"The
current plan is to keep these jumper cables in place, so that there is a
commanding path that is assured to reclose the hooks after the new Soyuz
arrives at the station on October 10," Navias explained. "They'll
bring up some spare parts and do some troubleshooting so that this problem is
alleviated in the future."

The module
is just over 13 feet (4 meters) long, about 8 feet (2.5 meters) wide and weighs
about 8,000 pounds (17,636 kg). It has about 380 cubic feet (10.7 cubic meters)
of living space inside.

While trying
to identify the cause of the docking problem, Russian cosmonaut Fyodor
Yurchikhin found a small star-shaped gear with two broken teeth, though it was
unclear what role the broken part may have played, if any, in the evening's
troubles.

Yurchikhin
and two NASA astronauts ? Douglas Wheelock and Shannon Walker ? are remaining
behind on the International Space Station to complete their own space mission
later this year.

Graphic ? Inside and Out: The
International Space Station

Video: Astronaut Describes Riding
Home on a Rocket

Gallery - Soyuz Spaceship's Snowy
Landing

NASA will
broadcast the upcoming Soyuz spacecraft undocking and landing live on NASA TV.